Electrochemical reaction device

ABSTRACT

An electrochemical reaction device includes: a first electrolytic solution tank including first and second storage parts storing first and second electrolytic solutions containing carbon dioxide and water respectively; reduction and oxidation electrodes immersed in the first and second electrolytic solutions respectively; a generator connected to the reduction and oxidation electrodes; a second electrolytic solution tank including a third storage part storing a third electrolytic solution containing carbon dioxide; and a flow path connecting the first and third storage parts. The third electrolytic solution is lower in temperature than the first electrolytic solution.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2016-032480, filed on Feb. 23, 2016; theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to an electrochemicalreaction device.

BACKGROUND

Artificial photosynthesis technology of electrochemically convertingsunlight into a chemical substance in imitation of photosynthesis ofplants is under development from viewpoints of energy problem andenvironmental problem. This is because, for example, this technologymakes it possible to obtain sufficient energy even if a chemicalsubstance produced by the conversion from sunlight in a land which is oflow utility value and not used for the production of plants, such as,for example, a desert is transported to a distant place. Convertingsunlight to a chemical substance to store it in a cylinder or a tank isadvantageous in that it costs lower for energy storage and has a lessstorage loss than converting sunlight to electricity to store it instorage batteries.

As a photoelectrochemical reaction device that electrochemicallyconverts sunlight to a chemical substance, there has been known, forexample, a two-electrode type device that includes an electrode having areduction catalyst for reducing carbon dioxide (CO₂) and an electrodehaving an oxidation catalyst for oxidizing water (H₂O), these electrodesbeing immersed in water in which carbon dioxide is dissolved. In thiscase, the electrodes are electrically connected to each other via anelectric wire or the like. The electrode having the oxidation catalystoxidizes H₂O using light energy to produce oxygen (½O₂) and obtains apotential. The electrode having the reduction catalyst obtains thepotential from the electrode that causes the oxidation reaction, therebyreducing the carbon dioxide to produce formic acid (HCOOH) or the like.Such two-stage excitation for obtaining the reduction potential of thecarbon dioxide makes the two-electrode type device low in conversionefficiency from the sunlight to the chemical energy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating a structure example of anelectrochemical reaction device.

FIG. 2 is a schematic view illustrating another structure example of theelectrochemical reaction device.

FIG. 3 is a schematic view illustrating a structure example of aphotoelectric conversion cell.

FIG. 4 is a schematic view illustrating another structure example of theelectrochemical reaction device.

FIG. 5 is a schematic view illustrating another structure example of theelectrochemical reaction device.

FIG. 6 is a schematic view illustrating another structure example of theelectrochemical reaction device.

FIG. 7 is a schematic view illustrating another structure example of theelectrochemical reaction device.

DETAILED DESCRIPTION

An electrochemical reaction device of an embodiment includes: a firstelectrolytic solution tank including a first storage part storing afirst electrolytic solution containing carbon dioxide and a secondstorage part storing a second electrolytic solution containing water; areduction electrode immersed in the first electrolytic solution; anoxidation electrode immersed in the second electrolytic solution; agenerator connected to the reduction electrode and the oxidationelectrode; a second electrolytic solution tank including a third storagepart storing a third electrolytic solution containing carbon dioxide;and a flow path connecting the first storage part and the third storagepart. A temperature of the third electrolytic solution is lower than atemperature of the first electrolytic solution.

Embodiments will be hereinafter described with reference to thedrawings. The drawings are schematic, and for example, the sizes such asthe thickness and width of each constituent element may differ from theactual sizes of the constituent element. In the embodiments,substantially the same constituent elements are denoted by the samereference signs and a description thereof will be omitted in some case.In this specification, the term “connect” not only means “directlyconnect” but also may include the meaning of “indirectly connect”.

FIG. 1 is a schematic view illustrating a structure example of anelectrochemical reaction device. As illustrated in FIG. 1, theelectrochemical reaction device includes an electrolytic solution tank11, an electrolytic solution tank 12, a reduction electrode 31, anoxidation electrode 32, a photoelectric conversion body 33, an ionexchange membrane 4, a flow path 51, and a flow path 52.

The electrolytic solution tank 11 has a storage part 111 and a storagepart 112. The electrolytic solution tank 11 is not limited to have aparticular shape and may have any three-dimensional shape having acavity serving as the storage part.

The storage part 111 stores an electrolytic solution 21 containing asubstance to be reduced. The substance to be reduced is a substance thatundergoes a reduction reaction to be reduced. The substance to bereduced contains, for example, carbon dioxide. Further, the substance tobe reduced may contain hydrogen ions. Changing an amount of watercontained in the electrolytic solution 21 or changing electrolyticsolution components can change reactivity to change selectivity of thesubstance to be reduced and a ratio of a produced chemical substance.

The storage part 112 stores an electrolytic solution 22 containing asubstance to be oxidized. The substance to be oxidized is a substancethat undergoes an oxidation reaction to be oxidized. The substance to beoxidized is, for example, water, or an organic matter such as alcohol oramine, or an inorganic oxide such as iron oxide. The electrolyticsolution 22 may contain the same substance as that contained in theelectrolytic solution 21. In this case, the electrolytic solution 21 andthe electrolytic solution 22 may be regarded as one electrolyticsolution.

The electrolytic solution 22 preferably has higher pH than pH of theelectrolytic solution 21. This facilitates the migration of hydrogenions, hydroxide ions, and the like. Further, al quid junction potentialdue to the difference in pH enables effective progress of anoxidation-reduction reaction.

The electrolytic solution tank 12 has a storage part 113 storing anelectrolytic solution 23. The electrolytic solution 23 contains carbondioxide, for instance. The electrolytic solution tank 12 has a functionas a reduction catalyst absorber. The temperature of the electrolyticsolution 23 is lower than the temperature of the electrolytic solution21.

The reduction electrode 31 is immersed in the electrolytic solution 21.The reduction electrode 31 contains a reduction catalyst for thesubstance to be reduced, for instance. A compound produced by thereduction reaction differs depending on, for example, the kind of thereduction catalyst. For example, the compound produced by the reductionreaction is: a carbon compound such as carbon monoxide (CO), formic acid(HCOOH), methane (CH₄), methanol (CH₃OH), ethane (C₂H₆), ethylene(C₂H₄), ethanol (C₂H₅OH), formaldehyde (HCHO), or ethylene glycol; orhydrogen. The compound produced by the reduction reaction may berecovered through a product flow path, for instance. In this case, theproduct flow path is connected to, for example, the storage part 111.The compound produced by the reduction reaction may be recovered throughanother flow path.

The reduction electrode 31 may have a structure in a thin film form, alattice form, a granular form or a wire form, for instance. Thereduction electrode 31 does not necessarily contain the reductioncatalyst. A reduction catalyst provided separately from the reductionelectrode 31 may be electrically connected to the reduction electrode31.

The oxidation electrode 32 is immersed in the electrolytic solution 22.The oxidation electrode 32 contains an oxidation catalyst for thesubstance to be oxidized, for instance. A compound produced by theoxidation reaction differs depending on, for example, the kind of theoxidation catalyst. Examples of the compound produced by the oxidationreaction include hydrogen ions. The compound produced by the oxidationreaction may be recovered through a product flow path, for instance. Inthis case, the product flow path is connected to, for example, thestorage part 112. The compound produced by the oxidation reaction may berecovered through another flow path.

The oxidation electrode 32 may have a structure in a thin film form, alattice form, a granular form, or a wire form, for instance. Theoxidation electrode 32 does not necessarily contain the oxidationcatalyst. An oxidation catalyst provided separately from the oxidationelectrode 32 may be electrically connected to the oxidation electrode32.

In a case where the oxidation electrode 32 is stacked and immersed inthe electrolytic solution 22, and where light is radiated to thephotoelectric conversion body 33 through the oxidation electrode 32 tocause the oxidation-reduction reaction, the oxidation electrode 32 needsto have a light transmitting property. Light transmittance of theoxidation electrode 32 is preferably, for example, at least 10% or more,more preferably 30% or more of an irradiation amount of the lightirradiating the oxidation electrode 32.

This is not restrictive, and the photoelectric conversion body 33 may beirradiated with the light through the reduction electrode 31, forinstance.

The photoelectric conversion body 33 has a face 331 electricallyconnected to the reduction electrode 31 and a face 332 electricallyconnected to the oxidation electrode 32. In FIG. 1, the face 331 and thereduction electrode 31, and the face 332 and the oxidation electrode 32are connected by heat transfer members such as wiring lines having aheat transfer property, for instance. Connecting the photoelectricconversion body to the reduction electrode or the oxidation electrode bythe wiring line or the like is advantageous as a system, sinceconstituent elements are separated according to the function. Thephotoelectric conversion body 33 may be disposed outside theelectrolytic solution tank 11. Incidentally, the photoelectricconversion body 33 does not necessarily have to be provided. Anothergenerator may be connected to the oxidation electrode 32 and thereduction electrode 31. The generator is not limited to thephotoelectric conversion element having the photoelectric conversionbody. Examples of the generator include a system power supply, a storagebattery, or the renewable energy such as the wind power, water power,and the geothermal power.

The photoelectric conversion body 33 has a function of separatingelectric charges when given energy of the irradiating light such assunlight. Electrons and holes generated by the charge separation migrateto the reduction electrode side and the oxidation electrode siderespectively. Consequently, the photoelectric conversion body 33 cangenerate an electromotive force. As the photoelectric conversion body33, a pn-junction or pin-junction photoelectric conversion body isusable, for instance. The photoelectric conversion body 33 may be fixedto the electrolytic solution tank 11, for instance. Incidentally, thephotoelectric conversion body 33 may be composed of a stack of aplurality of photoelectric conversion layers.

The reduction electrode 31, the oxidation electrode 32, and thephotoelectric conversion body 33 may be different in size.

The ion exchange membrane 4 is disposed so as to separate the storagepart 111 and the storage part 112. Examples of the ion exchange membrane4 include Neosepta (registered trademark) manufactured by ASTOMCorporation, Selemion (registered trademark) and Aciplex (registeredtrademark) manufactured by Asahi Glass Co. Ltd., fumasep (registeredtrademark) and fumapem (registered trademark) manufactured by FumatechGmbH, Nafion (registered trademark), which is a fluorocarbon resinproduced through polymerization of sulfonated tetrafluoroethylene,manufactured by Du Pont, Lewabrane (registered trademark) manufacturedby LANXESS, IONSEP (registered trademark) manufactured by IONTECH,Mustang (registered trademark) manufactured by Pall Corporation, ralex(registered trademark) manufactured by MEGA a.s., and Gore-Tex(registered trademark) manufactured by W. L. Gore & Associates. The ionexchange membrane 4 may be formed of a film having a hydrocarbon basicskeleton or for anion exchange, may be formed of a film having an aminegroup. Incidentally, the ion exchange membrane 4 does not necessarilyhave to be provided.

The flow path 51 and the flow path 52 have a function as electrolyticsolution flow paths to distribute the electrolytic solutions. Theirfunction is not limited to this, and the electrolytic solutions and theproducts by the oxidation-reduction reaction may be distributed throughthe flow path 51 and the flow path 52. For the electrolytic solutiontanks 11, 12 and the flow paths 51, 52, materials that transmit lightmay be used, for instance.

The flow path 51 connects the storage part 111 and the storage part 113.The ions and other substances contained in the electrolytic solution 21can move to the electrolytic solution tank 12 through the flow path 51.

The flow path 52 connects the storage part 111 and the storage part 113.Ions and other substances contained in the electrolytic solution 23 canmove to the electrolytic solution tank 11 through the flow path 52.

The shape of the flow path 51 and the flow path 52 is not limited to aparticular shape, provided that they have a shape having a cavityallowing the electrolytic solutions to flow therethrough, such as a pipeshape. The electrolytic solution of at least one of the flow path 51 andthe flow path 52 may be circulated by a circulation pump. At least partof the electrolytic solution 21 moves to the storage part 113 throughthe flow path 51, for instance. At least part of the electrolyticsolution 23 moves to the storage part 111 through the flow path 52, forinstance. The arrows illustrated in FIG. 1 indicate circulationdirections of the electrolytic solutions.

Next, an operation example of the electrochemical reaction deviceillustrated in FIG. 1 will be described. When light is incident on thephotoelectric conversion body 33, the photoelectric conversion body 33generates photoexcited electrons and holes. At this time, thephotoexcited electrons gather to the reduction electrode 31 and theholes gather to the oxidation electrode 32. Consequently, theelectromotive force is generated in the photoelectric conversion body33. As the light, sunlight is preferable, but light of a light emittingdiode, an organic EL, or the like may be incident on the photoelectricconversion body 33.

The following describes a case where electrolytic solutions containingwater and carbon dioxide are used as the electrolytic solution 21 andthe electrolytic solution 22 and carbon monoxide is produced. Around theoxidation electrode 32, as expressed by the following formula (1), thewater undergoes an oxidation reaction and loses electrons, so thatoxygen and hydrogen ions are produced. At least one of the producedhydrogen ions migrates to the storage part 111 through the ion exchangemembrane 4.

2H₂O→4H⁺+O₂+4e ⁻  (1)

Around the reduction electrode 31, as expressed by the following formula(2), the carbon dioxide undergoes a reduction reaction and the hydrogenions react with the carbon dioxide while receiving the electrons, sothat carbon monoxide and water are produced. Further, in addition to thecarbon monoxide, hydrogen is produced by the hydrogen ions receiving theelectrons as expressed by the following formula (3). At this time, thehydrogen may be produced simultaneously with the carbon monoxide.

CO₂+2H⁺+2e⁻→CO+H₂O   (2)

2H⁺+2e⁻→H₂   (3)

The photoelectric conversion body 33 needs to have an open-circuitvoltage equal to or more than a potential difference between a standardoxidation-reduction potential of the oxidation reaction and a standardoxidation-reduction potential of the reduction reaction. For example,the standard oxidation-reduction potential of the oxidation reaction inthe formula (1) is 1.23 [V]. The standard oxidation-reduction potentialof the reduction reaction in the formula (2) is −0.03 [V]. The standardoxidation-reduction potential of the reaction in the formula (3) is 0 V.In this case, the open-circuit voltage needs to be 1.26 [V] or more inthe reactions of the formula (1) and the formula (2).

The open-circuit voltage of the photoelectric conversion body 33 ispreferably higher than the potential difference between the standardoxidation-reduction potential of the oxidation reaction and the standardoxidation-reduction potential of the reduction reaction by a value ofovervoltages or more. For example, the overvoltages of the oxidationreaction in the formula (1) and the reduction reaction in the formula(2) are both 0.2 [V]. The open-circuit voltage is preferably 1.66 [V] ormore in the reactions of the formula (1) and the formula (2). Similarly,the open-circuit voltage is preferably 1.63 [V] or more in the reactionsof the formula (1) and the formula (3).

The reduction reactions of hydrogen ions and carbon dioxide arereactions consuming hydrogen ions. This means that a small amount of thehydrogen ions results in low efficiency of the reduction reaction. So,the electrolytic solution 21 and the electrolytic solution 22 preferablyhave different hydrogen ion concentrations so that the concentrationdifference facilitates the migration of the hydrogen ions. Theconcentration of anions (for example, hydroxide ions) may be madedifferent between the electrolytic solution 21 and the electrolyticsolution 22.

Reaction efficiency of the formula (2) varies depending on theconcentration of the carbon dioxide dissolved in the electrolyticsolution. The higher the concentration of the carbon dioxide, the higherthe reaction efficiency, and as the former is lower, the latter islower. Since solubility of the carbon dioxide is low, it is difficult toincrease the concentration of the carbon dioxide in the electrolyticsolution. The reaction efficiency of the formula (2) also variesdepending on the concentration of hydrogen carbonate ions or carbonateions. However, the concentration of hydrogen carbonate ions or theconcentration of carbonate ions can be adjusted by an increase of theelectrolytic solution concentration or the adjustment of pH and thus ismore easily adjusted than the carbon dioxide concentration.Incidentally, even if the ion exchange membrane is provided between theoxidation electrode and the reduction electrode, carbon dioxide gas,carbonate ions, hydrogen carbonate ions, and so on pass through the ionexchange membrane 4 and thus it is difficult to completely preventperformance deterioration.

A possible method to increase the carbon dioxide concentration may be,for example, a method of blowing the carbon dioxide directly to theelectrolytic solution tank 11. However, in a case where the reductionproduct is gaseous carbon monoxide or the like, the carbon dioxide gasand the carbon monoxide gas need to be separated. This results in a costincrease due to the complication of the device, and an energy loss dueto the need for energy for the separation.

The electrochemical reaction device of this embodiment includes thefirst electrolytic solution tank used for the oxidation-reductionreaction and the second electrolytic solution tank connected to thefirst electrolytic solution tank. The temperature of the electrolyticsolution stored in the storage part of the second electrolytic solutiontank is lower than the temperature of the electrolytic solution storedin the storage part of the first electrolytic solution tank. Forexample, cooling the storage part in the second electrolytic solutiontank can make the temperature of the electrolytic solution stored in thesecond electrolytic solution tank lower than the temperature of theelectrolytic solution stored in the storage part of the firstelectrolytic solution tank. Solubility of the carbon dioxide in thesecond electrolytic solution tank is higher than solubility of thecarbon dioxide in the first electrolytic solution tank.

It is possible to increase the carbon dioxide concentration in thesecond electrolytic solution tank also by making a pressure applied tothe electrolytic solution 23 higher than a pressure applied to theelectrolytic solution 21. In this case, the pressure of the storage partof the second electrolytic solution tank may be set higher than thepressure of the storage part of the first electrolytic solution tank.Further, a pressure regulator may be provided in the flow path 52.

By supplying the first electrolytic solution tank with the electrolyticsolution whose carbon dioxide concentration has been adjusted high inthe second electrolytic solution tank, it is possible to increase thecarbon dioxide concentration of the electrolytic solution stored in thefirst electrolytic solution tank. This can improve efficiency of thereduction reaction.

If the storage parts of the first electrolytic solution tank are cooled,the reactions by the catalysts deteriorate and accordingly reactionefficiency tends to lower. If the pressure is applied to the storageparts of the first electrolytic solution tank, pressure resistance ofthe electrolytic solution tank needs to be increased, leading to anincreased cost and a complicated structure. Further, the increase of thepressure resistance worsens maintainability, for example, making thechange of the electrodes troublesome.

For a reduction of a supply amount of the carbon dioxide and efficientabsorption of the carbon dioxide in the electrolytic solution, aninterval between bubbles of the carbon dioxide passing through theelectrolytic solution needs to be wide. However, the interval of thebubbles becomes short when the carbon dioxide concentration isincreased, allowing the downsizing of the electrolytic solution. Acooling temperature is preferably equal to or lower than the temperatureof the electrolytic solution in the first electrolytic solution tank,for instance. If the temperature of the electrolytic solution isincreased by the oxidation-reduction reaction, the cooling temperatureis preferably not lower than the room temperature nor higher than thetemperature of the electrolytic solution of the first electrolyticsolution tank. The cooling temperature is more preferably not lower thana temperature at which the electrolytic solution freezes nor more thanthe temperature of the electrolytic solution.

The temperature of the electrolytic solution in the first electrolyticsolution tank is preferably higher than the freezing point. For example,in a case where the electrolytic solution contains ions such aspotassium ions or sodium ions for the purpose of increasing anabsorption amount of carbon dioxide, increasing the concentrations ofcarbon dioxide ions and HCO₃ ions, and increasing solution resistance ofthe electrolytic solution, the electrolytic solution does not freeze at° C. However, extreme cooling requires a large cooler, leading to a costincrease and an energy loss, and thus the temperature of theelectrolytic solution is preferably 0° C. or higher in some case.Further, 5° C. or higher or 10° C. or higher is preferable in some casebecause of a concern about an energy loss of the whole electrochemicalreaction device and reaction deterioration due to the extreme cooling ofthe electrolytic solution.

Temperature regulators may be provided in the electrolytic solutiontanks 11 12 or the flow paths 51, 52 to impede the deterioration ofreaction efficiency due to a temperature decrease of the electrolyticsolution. Adjusting the temperature by the temperature regulatorimproves the reaction efficiency. For example, a cooler may be providedin the flow path 51 and a heater may be provided in the flow path 52Further since even a temperature difference of several ° C. can producethe effect, irradiating the electrolytic solution flow path between thefirst electrolytic solution tank and the second electrolytic solutiontank or irradiating the electrolytic solution tank with sunlight to heatit is efficient owing to the use of natural energy. Further, in alater-described case where the primary reaction is caused by electricenergy generated by the conversion from sunlight, heat energy and lightenergy of the sunlight can be efficiently used, resulting in furtherimprovement of efficiency.

Structure examples of the constituent elements in the electrochemicalreaction device will be further described. As a water-containingelectrolytic solution usable as the electrolytic solution, an aqueoussolution containing a desired electrolyte is usable, for instance. Thissolution is preferably an aqueous solution that promotes the oxidationreaction of water. Examples of the aqueous solution containing theelectrolyte include aqueous solutions containing phosphoric acid ions(PO₄ ²⁻), boric acid ions (BO₃ ³⁻), sodium ions (Na⁺), potassium ions(K⁺), calcium ions (Ca²⁺), lithium ions (Li⁺), cesium ions (Cs⁺),magnesium ions (Mg²⁺), chloride ions (Cl⁻), or hydrogen carbonate ions(HCO₃ ⁻).

Examples of an electrolytic solution containing carbon dioxide usable asthe electrolytic solution include aqueous solutions containing LiHCO₃,NaHCO₃, KHCO₃, CsHCO₃, phosphoric acid, or boric acid. The electrolyticsolution containing carbon dioxide may contain alcohol such as methanol,ethanol, or acetone. The electrolytic solution containing water may bethe same as the electrolytic solution containing carbon dioxide.However, an absorption amount of carbon dioxide in the electrolyticsolution containing carbon dioxide is preferably high. So, as theelectrolytic solution containing carbon dioxide, a solution differentfrom the electrolytic solution containing water may be used. Theelectrolytic solution containing carbon dioxide is preferably anelectrolytic solution that lowers a reduction potential of carbondioxide, has high ion conductivity, and contains a carbon dioxideabsorbent that absorbs carbon dioxide.

As the aforesaid electrolytic solution, an ionic liquid that containssalts of cations such as imidazolium ions or pyridinium ions and anionssuch as BF₄ ⁻ or PF₆ ⁻ and is in a liquid state in a wide temperaturerange, or its aqueous solution is usable, for instance. Other examplesof the electrolytic solution include solutions of amine such asethanolamine, imidazole, and pyridine, and aqueous solutions thereof.Examples of the amine include primary amine, secondary amine, andtertiary amine. These electrolytic solutions may be high in ionconductivity, have a property of absorbing carbon dioxide, and have aproperty of lowering reduction energy.

Examples of the primary amine include methylamine, ethylamine,propylamine, butylamine, pentylamine, and hexylamine. Hydrocarbons ofthe amine may be substituted by alcohol, halogen, or the like. Examplesof the amine whose hydrocarbons are substituted include methanolamine,ethanolamine, and chloromethyl amine. Further, an unsaturated bond mayexist. The same thing can be said for hydrocarbons of the secondaryamine and the tertiary amine.

Examples of the secondary amine include dimethylamine, diethylamine,dipropylamine, dibutylamine, dipentylamine, dihexylamine,dimethanolamine, diethanolamine, and dipropanolamine. The substitutedhydrocarbons may be different. This also applies to the tertiary amine.Examples of the amine having different hydrocarbons includemethylethylamine and methylpropylamine.

Examples of the tertiary amine include trimethylamine, triethylamine,tripropylamine, tributylamine, trihexylamine, trimethanolamine,triethanolamine, tripropanolamine, tributanolamine, tripropanolamine,triexanolamine, methyldiethylamine, and methyldipropylamine.

Examples of the cations of the ionic liquid include

-   1-ethyl-3-methylimidazolium ions, 1-methyl-3-propylimidazolium ions,    1-butyl-3-methylimidazole ions, 1-methyl-3-pentylimidazolium ions,    and 1-hexyl-3-methylimidazolium ions.

A second place of imidazolium ions may be substituted. Examples of thecations in which the second place of the imidazolium ions is substitutedinclude

-   1-ethyl-2,3-dimethylimidazolium ions,    1-2-dimethyl-3-propylimidazolium ions,    1-butyl-2,3-dimethylimidazolium ions,    1,2-dimethyl-3-pentylimidazolium ions, and    1-hexyl-2,3-dimethylimidazolium ions.

Examples of pyridinium ions include methylpyridinium, ethylpyridinium,propylpyridinium, butylpyridinium, pentylpyridinium, andhexylpyridinium. In both of the imidazolium ions and the pyridiniumions, an alkyl group may be substituted, or an unsaturated bond mayexist.

Examples of the anions include fluoride ions, chloride ions, bromideions, iodide ions, BF₄ ⁻, PF₆ ⁻, CF₃COO⁻, CF₃SO₃ ⁻, NO₃ ⁻, SCN⁻,(CF₃SO₂)₃C⁻, bis(trifluoromethoxysulfonyl)imide,bis(trifluoromethoxysulfonyl)imide, andbis(perfluoroethylsulfonyl)imide. Dipolar ions in which the cations andthe anions of the ionic liquid are coupled by hydrocarbons may be used.Incidentally, a buffer solution such as a potassium phosphate solutionmay be supplied to the storage parts 111, 112.

FIG. 2 is a view illustrating another example of the electrochemicalreaction device. The electrochemical reaction device illustrated in FIG.2 is different from the electrochemical reaction device illustrated inFIG. 1 in that the reduction electrode 31, the oxidation electrode 32,and the photoelectric conversion body 33 are stacked. The reductionelectrode 31 is in contact with the face 331 and the oxidation electrode32 is in contact with the face 332. In this case, a stack including thereduction electrode 31, the oxidation electrode 32, and thephotoelectric conversion body 33 is also called a photoelectricconversion cell. The photoelectric conversion cell penetrates throughthe ion exchange membrane 4 and is immersed in the electrolytic solution21 and the electrolytic solution 22.

FIG. 3 is a schematic cross-sectional view illustrating a structureexample of the photoelectric conversion cell. The photoelectricconversion cell illustrated in FIG. 3 includes a conductive substrate30, the reduction electrode 31, the oxidation electrode 32, thephotoelectric conversion body 33, a light reflective body 34, a metaloxide body 35, and a metal oxide body 36.

The conductive substrate 30 is in contact with the reduction electrode31. The conductive substrate 30 may be regarded as part of the reductionelectrode. Examples of the conductive substrate 30 include a substratecontaining at least one or more of Cu, Al, Ti, Ni, Fe, and Ag. Forexample, a stainless steel substrate containing stainless steel such asSUS may be used. The conductive substrate 30 is not limited to the aboveand may be formed of a conductive resin. Alternatively, the conductivesubstrate 30 may be constituted by a substrate of a semiconductor suchas Si or Ge. Further, a resin film or the like may be used as theconductive substrate 30. For example, the film usable as the ionexchange membrane 4 may be used as the conductive substrate 30.

The conductive substrate 30 has a function as a support. The conductivesubstrate 30 may be disposed so as to separate the storage part 111 andthe storage part 112. The presence of the conductive substrate 30 canimprove mechanical strength of the photoelectric conversion cell.Further, the conductive substrate 30 may be regarded as part of thereduction electrode 31. Further, the conductive substrate 30 does notnecessarily have to be provided.

The reduction electrode 31 preferably contains a reduction catalyst. Thereduction electrode 31 may contain both a conductive material and thereduction catalyst. Examples of the reduction catalyst include amaterial that reduces activation energy for reducing hydrogen ions orcarbon dioxide. In other words, a material that lowers the overvoltageswhen hydrogen and a carbon compound are produced by the reductionreactions of hydrogen ions and carbon dioxide is usable. For example, ametal material or a carbon material is usable. For example, in theproduction of hydrogen, a metal such as platinum or nickel, or an alloycontaining this metal is usable as the metal material. In the reductionreaction of carbon dioxide, a metal such as gold, aluminum, copper,silver, platinum, palladium, or nickel, or an alloy containing thismetal is usable. As the carbon material, graphene, carbon nanotube(CNT), fullerene, or ketjen black is usable, for instance. The reductioncatalyst is not limited to these, and may be, for example, a metalcomplex such as a Ru complex or a Re complex, or an organic moleculehaving an imidazole skeleton or a pyridine skeleton, or may be a mixtureof a plurality of materials.

The oxidation electrode 32 preferably contains an oxidation catalyst.The oxidation electrode 32 may contain both a conductive material andthe oxidation catalyst. Examples of the oxidation catalyst include amaterial that reduces activation energy for oxidizing water. In otherwords, a material that lowers the overvoltage when oxygen and hydrogenions are produced by the oxidation reaction of water is usable. Examplesthereof include iridium, platinum, cobalt, and manganese. Further, asthe oxidation catalyst, a binary metal oxide, a ternary metal oxide, ora quaternary metal oxide is usable, for instance. Examples of the binarymetal oxide include manganese oxide (Mn—O), iridium oxide (Ir—O), nickeloxide (Ni—O), cobalt oxide (Co—O), iron oxide (Fe—O), tin oxide (Sn—O),indium oxide (In—O), and ruthenium oxide (Ru—O). Examples of the ternarymetal oxide include Ni—Co—O, La—Co—O, Ni—La—O, and Sr—Fe—O. Examples ofthe quaternary metal oxide include Pb—Ru—Ir—O and La—Sr—Co—O. Theoxidation catalyst is not limited to these, and may be a metal complexsuch as a Ru complex or a Fe complex, or a mixture of a plurality ofmaterials.

At least one of the reduction electrode 31 and the oxidation electrode32 may have a porous structure. Examples of a material usable for theelectrode having the porous structure include, in addition to theabove-listed materials, carbon black such as ketjen black and VULCANXC-72, activated carbon, and metal fine powder. The porous structure canincrease the area of an active surface contributing to theoxidation-reduction reaction and thus can increase conversionefficiency.

In a case where an electrode reaction with a low current density iscaused using relatively low irradiation energy of light, the catalystmaterial can be selected from a wide range of options. Accordingly, itis easy to cause the reaction using, for example, a ubiquitous metal,and it is also relatively easy to obtain selectivity of the reaction. Onthe other hand, in a case where the photoelectric conversion body 33 isnot disposed in the electrolytic solution tank 11 and is electricallyconnected to at least one of the reduction electrode 31 and theoxidation electrode 32 by, for example, a wiring line, the electrodearea is usually decreased due to a reason such as the downsizing of theelectrolytic solution tank, and the reaction is sometimes caused with ahigh current density. In this case, a noble metal is preferably used asthe catalyst.

The photoelectric conversion body 33 has a stacked structure of aphotoelectric conversion layer 33 x, a photoelectric conversion layer 33y, and a photoelectric conversion layer 33 z. The number of the stackedphotoelectric conversion layers is not limited to that in FIG. 3.

The photoelectric conversion layer 33 x has, for example, an n-typesemiconductor layer 331 n containing n-type amorphous silicon, an i-typesemiconductor layer 331 i containing intrinsic amorphous silicongermanium, and a p-type semiconductor layer 331 p containing p-typemicrocrystalline silicon. The i-type semiconductor layer 331 i is alayer that absorbs light in a short wavelength range including 400 nm,for instance. Accordingly, in the photoelectric conversion layer 33 x,charge separation is caused by energy of light in the short wavelengthrange.

The photoelectric conversion layer 33 y has, for example, an n-typesemiconductor layer 332 n containing n-type amorphous silicon, an i-typesemiconductor layer 332 i containing intrinsic amorphous silicongermanium, and a p-type semiconductor layer 332 p containing p-typemicrocrystalline silicon. The i-type semiconductor layer 332 i is alayer that absorbs light in an intermediate wavelength range including600 nm, for instance. Accordingly, in the photoelectric conversion layer33 y, charge separation is caused by energy of light in the intermediatewavelength range.

The photoelectric conversion layer 33 z has, for example, an n-typesemiconductor layer 333 n containing n-type amorphous silicon, an i-typesemiconductor layer 333 i containing intrinsic amorphous silicon, and ap-type semiconductor layer 333 p containing p-type microcrystallinesilicon. The i-type semiconductor layer 333 i is a layer that absorbslight in a long wavelength range including 700 nm, for instance.Accordingly, in the photoelectric conversion layer 33 z, chargeseparation is caused by energy of light in the long wavelength range.

The p-type semiconductor layers or the n-type semiconductor layers eachcan be formed of, for example, a semiconductor material to which anelement that is to be a donor or an acceptor is added. Incidentally, inthe photoelectric conversion layer, as the semiconductor layers, thesemiconductor layers containing silicon, germanium, or the like areused, but the semiconductor layers are not limited to these, and maybecompound semiconductor layers, for instance. As the compoundsemiconductor layers, semiconductor layers containing, for example,GaAs, GaInP, AlGaInP, CdTe, or CuInGaSe are usable, for instance.Further, layers containing a material such as TiO₂ or WO₃ may be used,provided that photoelectric conversion is possible. Further, thesemiconductor layers each may be monocrystalline, polycrystalline, oramorphous. Further, the photoelectric conversion layer may include azinc oxide layer.

The light reflective body 34 is between the conductive substrate 30 andthe photoelectric conversion body 33. Examples of the light reflectivebody 34 include a distributed Bragg reflection layer composed of a stackof metal layers or semiconductor layers, for instance. Owing to thepresence of the light reflective body 34, light that cannot be absorbedby the photoelectric conversion body 33 can be reflected to enter one ofthe photoelectric conversion layer 33 x to the photoelectric conversionlayer 33 z, enabling to enhance conversion efficiency from light to achemical substance. As the light reflective body 34, a layer of a mealsuch as Ag, Au, Al, or Cu or an alloy containing at least one of thesemetals is usable, for instance.

The metal oxide body 35 is between the light reflective body 34 and thephotoelectric conversion body 33. The metal oxide body 35 has a functionof enhancing light reflectivity by adjusting an optical distance, forinstance. For the metal oxide body 35, a material capable of ohmiccontact with the n-type semiconductor layer 331 n is preferable used. Asthe metal oxide body 35, a layer of a light-transmissive metal oxidesuch as, for example, indium tin oxide (ITO), zinc oxide (ZnO),fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (AZO), orantimony-doped tin oxide (ATO) is usable.

The metal oxide body 36 is between the oxidation electrode 32 and thephotoelectric conversion body 33. The metal oxide body 36 may bedisposed on a surface of the photoelectric conversion body 33. The metaloxide body 36 has a function as a protective layer preventing thephotoelectric conversion cell from being broken by the oxidationreaction. The presence of the metal oxide body 36 can prevent thecorrosion of the photoelectric conversion body 33 to extend the life ofthe photoelectric conversion cell. Incidentally, the metal oxide body 36does not necessarily have to be provided.

As the metal oxide body 36, a dielectric thin film of TiO₂, ZrO₂, Al₂O₃,SiO₂, or HfO₂ is usable, for instance. The metal oxide body 36preferably has a thickness of 10 nm or less, further 5 nm or less. Thisis intended to obtain electrical conductivity by a tunnel effect. As themetal oxide body 36, a layer of a light transmissive metal oxide suchas, for example, indium tin oxide (ITO), zinc oxide (ZnO),fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (AZO), orantimony-doped tin oxide (ATO) may be used.

The metal oxide body 36 may have, for example, a stacked structure of ametal and a transparent conductive oxide, a composite structure of ametal and another conductive material, or a composite structure of atransparent conductive oxide and another conductive material. The abovestructure can decrease the number of parts, decrease the weight, andfacilitate the manufacture, enabling cost reduction. The metal oxidebody 36 may have functions as a protective layer, a conductive layer,and a catalyst layer.

In the photoelectric conversion cell illustrated in FIG. 3, a face ofthe n-type semiconductor layer 331 n opposite to its contact surfacewith the i-type semiconductor layer 331 i is a first face of thephotoelectric conversion body 33, and a face of the p-type semiconductorlayer 333 p opposite to its contact surface with the i-typesemiconductor layer 333 i is a second face. The photoelectric conversioncell illustrated in FIG. 3 has the stacked structure of thephotoelectric conversion layer 33 x to the photoelectric conversionlayer 33 z as described above and thus is capable of absorbing lights ina wide wavelength range of sunlight, enabling more efficient use ofenergy of sunlight. In this case, a high voltage can be obtained owingto the series connection of the photoelectric conversion bodies.

In FIG. 3, electrons and holes having undergone the charge separationcan be used as they are in the oxidation-reduction reaction, since theelectrodes are stacked on the photoelectric conversion body 33. Further,the photoelectric conversion body 33 and the electrodes need not beelectrically connected by wiring lines or the like. This enables ahigh-efficiency oxidation-reduction reaction.

The plural photoelectric conversion bodies may be electrically connectedin parallel. A dual junction or single-layer photoelectric conversionbody may be used. A stack of two photoelectric conversion bodies, orfour photoelectric conversion bodies or more may be used. A single-layerphotoelectric conversion layer may be used instead of the stack of theplural photoelectric conversion layers.

The electrochemical reaction device of this embodiment is a simplifiedsystem with a reduced number of parts owing to the integration of thereduction electrode, the oxidation electrode, and the photoelectricconversion body. This facilitates at least one of, for example,manufacture, installation, and maintenance. Further, this structureeliminates a need for wiring lines connecting the photoelectricconversion body to the reduction electrode and the oxidation electrode,achieving an increased light transmittance and an increasedlight-receiving area.

The photoelectric conversion body 33 is in contact with the electrolyticsolution, which may lead to its corrosion and the dissolving ofcorrosive products in the electrolytic solution to deteriorate theelectrolytic solution. A possible measure to prevent the corrosion maybe to provide a protective layer. However, components of the protectivelayer may dissolve in the electrolytic solution. Here, providing afilter such as a metal ion filter in the flow path or the electrolyticsolution tank hinders the deterioration of the electrolytic solution.

The electrochemical reaction device of this embodiment is an artsuitable as a measure for surplus power and is required to make good useof solar energy. In a case where illuminance of sunlight is high, whenthere is no surplus power, energy is obtained as much as possible, andwhen there is surplus energy, the energy is consumed by being used forcirculating the electrolytic solution. This enables efficient energy mixto increase the total energy utilization ratio. In a case where a buffersolution is used as the electrolytic solution, a small reaction amountalso results in a small pH change caused by the reaction. So, during anon-reaction period, the electrolytic solution is circulated to keep theelectrolytic solution components uniform, and during the reaction, thesupply of the electrolytic solution is restricted or stopped. This canprevent a decrease of total efficiency and reduce the cost. For example,preferably, the electrolytic solution is circulated using nighttime windpower or low-cost surplus power, and in the daytime, theoxidation-reaction reaction is caused, with the circulation of theelectrolytic solution being stopped or with the minimum supply amount ofthe electrolytic solution.

A structure example of the electrochemical reaction device is notlimited to that in FIG. 1. FIG. 4 is a schematic view illustratinganother example of the electrochemical reaction device. Theelectrochemical reaction device illustrated in FIG. 4 is different fromthe electrochemical reaction device illustrated in FIG. 1 at least inthat it further includes a separation tank 13, a separation tank 14, aflow path 53 to a flow path 55.

The separation tank 13 has a storage part 114 a storing an electrolyticsolution 24 and a gas-liquid separation membrane 114 b dividing thestorage part 114 a into a plurality of regions. The gas-liquidseparation membrane 114 b includes, for example, a hollow fiber membraneand so on. The hollow fiber membrane contains, for example, a siliconeresin, a fluorine-based resin (perfluoroalkoxyalkane (PFA), aperfluoroethylene propene copolymer (F E P), polytetrafluoroethylene(PTFE), an ethylene-tetrafluoroethylene copolymer (ETFE), polyvinylidenefluoride (PVDF), polychlorotrifluoroethylene (PCTFE), anethylene-chlorotrifluoroethylene copolymer (ECTFE)), or the like.

In the electrochemical reaction device illustrated in FIG. 4, part ofthe reduction product in the electrolytic solution tank 11 is extractedin the separation tank 13. An outer side of the gas-liquid separationmembrane 114 b (its surface side opposite to its contact surface withthe electrolytic solution 24) is pressure-reduced and the electrolyticsolution 24 containing a gaseous product passes through the gas-liquidseparation membrane 114 b, enabling the efficient separation of thegaseous product and carbon dioxide. In a case where the product is, forexample, carbon monoxide, only carbon monoxide gas can be separated bythe gas-liquid separation in the separation tank 13.

The flow path 51 is connected to the storage part 114 a. The flow path53 connects the storage part 113 and the storage part 114 a. Theseparation tank 14 has a storage part 115 a storing an electrolyticsolution 25 and a gas-liquid separation membrane 115 b dividing thestorage part 115 a into a plurality of regions. The flow path 54connects the storage part 112 and the storage part 115 a. The flow path55 connects the storage part 112 and the storage part 115 a. At leastpart of the electrolytic solution 22 is supplied to the storage part 115a through the flow path 54. At least part of the electrolytic solution25 is supplied to the storage part 112 through the flow path 55.Circulation pumps or the like may be provided in the flow path 54 andthe flow path 55.

An outer side of the gas-liquid separation membrane 115 b (its surfaceside opposite to its contact surface with the electrolytic solution 25)is pressure-reduced and the electrolytic solution containing a gaseousproduct passes through the gas-liquid separation membrane 115 b, so thatoxygen gas and dissolved oxygen can be separated similarly to the carbondioxide. It can be conceived to directly recover and use the oxygen gasgenerated in the electrolytic solution tank 11, but since the oxygen gasis dissolved in the electrolytic solution 22, it is difficult tocompletely recover the oxygen gas. Since the dissolved oxygendeteriorates performance of the oxide electrode, the dissolved oxygen isdesirably recovered in the form of gas. Unlike the gas separation in theelectrolytic solution tank 11, it is possible to recover gases generatedin a plurality of cells at a time. Accordingly, the total flow pathlength for the gas recovery is shortened, enabling a simplified system.In this case, by providing temperature regulators in the separation tank14 or the flow path 54 and the flow path 55 as in the electrolyticsolution tank 12 in order to efficiently recover the oxygen gas, it ispossible to efficiently separate oxygen from the electrolytic solution.

By providing a temperature regulator in the separation tank 13 or theflow path 51, it is possible to enhance separation efficiency of theproduct. For the complete gas separation, the dissolved gas in theelectrolytic solution is preferably removed as much as possible. Anagitator is preferably provided in the separation tank 13 to enhanceefficiency of removing the dissolved gas by temperature distribution orthe like.

A difference between the temperature of the electrolytic solution 24 inthe separation tank 13 and the temperature of the electrolytic solution21 in the electrolytic solution tank 11 may be not less than −10° C. normore than 10° C. Too high a temperature of the electrolytic solution 24in the separation tank 13 is likely to decrease the gas concentration ofthe product due to the vaporization of carbon dioxide dissolved in theelectrolytic solution 24. Excessive heating leads to efficiencydeterioration because of a large energy loss by the heating.

In a case where the product is a water-soluble liquid substance such asmethanol or ethanol, a separation method in the separation tank 13 maybe distillation or membrane separation, for instance. In this case, atemperature regulator is desirably provided to improve separationefficiency. The separation membrane may be zeolite, for instance. Heatespecially on an upstream side is large and thus is likely todeteriorate the total efficiency. To cope with this, providing a heatinsulator in the separation tank 13 can prevent the efficiencydeterioration.

In a case where an ion exchange membrane or a flow path is providedbetween the oxidation electrode and the reduction electrode in theelectrolytic solution tank, the electrolytic solution in contact withthe oxidation electrode may be different from the electrolytic solutionin contact with the reduction electrode. By the above structure, it ispossible to easily separate and extract oxygen being the reactionproduct in the oxidation side.

A suitable electrolytic solution differs depending on each catalyst, andby making the electrolytic solutions in contact with the catalyst layersdifferent, it is possible to improve efficiency. Furthermore, making pHon the oxidation side larger than that on the reduction side isadvantageous in that a liquid junction potential caused by the pHdifference can compensate for an insufficient potential of the reaction.

An electrochemical reaction device illustrated in FIG. 5 includes thestructure of the electrochemical reaction device illustrated in FIG. 4,a flow path 56, a cooler 61 a, a cooler 61 b, a heater 62 a, a heater 62b, a pump 71, and a pressure valve 72.

The flow path 56 is connected to the storage part of the electrolyticsolution tank 12. For example, the flow path 56 is connected to a carbondioxide generation source 80.

The cooler 61 a has a function of cooling the electrolytic solutionflowing in the flow path 56. The cooler 61 a may be disposed inside oroutside the flow path 56, for instance.

The cooler 61 b has a function of cooling the electrolytic solution 23.The cooler 61 b may be disposed inside or outside the storage part 113,for instance.

The heater 62 a has a function of heating the electrolytic solution 25.The heater 62 a may be disposed inside or outside the storage part 115a, for instance.

The heater 62 b has a function of heating the electrolytic solutionflowing in the flow path 54. The heater 62 b may be disposed inside oroutside the flow path 54, for instance.

The pump 71 has a function of promoting the supply of the electrolyticsolution from the storage part 114 a to the storage part 113. The pump71 is disposed inside or outside the flow path 53, for instance. Thepump 71 does not necessarily have to be provided.

The pressure valve 72 has a function of promoting the supply of theelectrolytic solution from the storage part 113 to the storage part 111.The pressure valve 72 is disposed inside or outside the flow path 52,for instance. Examples of the pressure valve 72 include an orifice valveand a pulse valve. The pressure valve 72 does not necessarily have to beprovided.

Heat exchange between the separation tank 13 and the separation tank 14may be performed. The heat exchange is possible by providing a heattransfer member 91 connecting, for example, the separation tank 13 andthe separation tank 14. The heat transfer member 91 may be provided soas to connect the storage part 114 a and the storage part 115 a, forinstance. Alternatively, a heat exchanger or the like may be separatelyconnected.

An electrochemical reaction device illustrated in FIG. 6 furtherincludes a cooler 61 c in addition to the structure illustrated in FIG.5, and does not include the separation tank 13.

The flow path 53 connects the storage part 111 and the storage part 113.The flow path 56 is connected to the storage part 111. The flow path 56connects, for example, the storage part 111 and the carbon dioxidegeneration source 80. The carbon dioxide generation source 80 may bedisposed inside or outside the electrochemical reaction device.

Heat exchange between the electrolytic solution tank 12 and theseparation tank 14 may be performed. The heat exchange is possible byproviding a heat transfer member 92 connecting, for example, theelectrolytic solution tank 12 and the separation tank 14.

The heat transfer member 92 may be provided so as to connect the flowpath 53 and the flow path 54, for instance. Alternatively, a heatexchanger or the like may be separately connected.

The cooler 61 c has a function of cooling the electrolytic solutionflowing in the flow path 53. The cooler 61 c is disposed inside oroutside the flow path 53, for instance.

The pump 71 has a function of promoting the supply of the electrolyticsolution from the storage part 113 to the storage part 111. The pump 71is disposed in the flow path 52, for instance.

The pressure valve 72 has a function of promoting the supply of theelectrolytic solution from the storage part 111 to the storage part 113.The pressure valve 72 is disposed inside or outside the flow path 52,for instance. Examples of the pressure valve 72 include an orifice valveand a pulse valve. Incidentally, the pressure valve 72 does notnecessarily have to be provided.

In the electrochemical reaction devices illustrated in FIG. 5 and FIG.6, the use of the coolers can facilitate lowing the temperature of theelectrolytic solution on the reduction side. Further, the use of theheaters can facilitate raising the temperature of the electrolyticsolution on the oxidation side. This can enhance reaction efficiency.

High-temperature carbon dioxide is generated in power plants,incinerators, and the like. The direct supply of the high-temperaturecarbon dioxide to the electrolytic solution tank 11 causes a temperatureincrease. The temperature increase is preferably reduced by providingthe cooler in the flow path 56 between the carbon dioxide generationsource 80 and the electrolytic solution tank 11. A cooler which coolsthe flow path by, for example, the atmospheric air, seawater, riverwater, or the like can also produce a sufficient effect.

It is possible to reduce an energy loss by supplying carbon dioxidepressurized in the carbon dioxide generation source 80 such as the powerplant or the incinerator to the electrolytic solution tank 11 or theelectrolytic solution tank 12 through the flow path without using a pumpor the like. A pressure regulator may be provided for pressurestabilization. Owing to the pressure regulator, carbon dioxide with astable pressure can be absorbed in the electrolytic solution. This canenhance stability of the whole device. Further, by improving efficiencyby performing voltage control across the reduction electrode and theoxidation electrode and temperature control and pressure control of theelectrochemical reaction device according to a supply amount and thetemperature of carbon dioxide from the electrolytic solution tank 11 andan operation signal of a carbon dioxide supply device, it is possible tomake the best use of performance of the device to improve theefficiency.

In a case where the separation tank 13 is heated, the use of heat of thecarbon dioxide generation source or the like for the heating reduces anenergy loss to improve efficiency. On the other hand, the use of heat ofthe high-temperature carbon dioxide gas supplied from the carbon dioxidegeneration source lowers the temperature of the carbon dioxide gassupplied to the electrolytic solution tank 12 to improve efficiency.

An electrochemical reaction device illustrated in FIG. 7 furtherincludes, in addition to the structure of the electrochemical reactiondevice illustrated in FIG. 6, a distiller 81 a, a reduction reactiondevice 81 b, and a flow path 57 connecting the storage part 113 and thereduction reaction device 81 b. The electrochemical reaction devicefurther includes a cooler 61 d instead of the cooler 61 c. Incidentally,it may include both the cooler 61 c and the cooler 61 d.

The cooler 61 d has a function of cooling the electrolytic solutionflowing in the flow path 52. The cooler 61 d is disposed inside oroutside the flow path 52, for instance.

The distiller 81 a has a function of distilling the product in thestorage part 113. The distiller 81 a is connected to the storage part113. The distiller 81 a is disposed on the electrolytic solution tank12, for instance. In the electrochemical reaction device illustrated inFIG. 7, efficiency can be improved since heat deprived of by thedistillation in the distiller 81 a and the high-temperature carbondioxide gas from the carbon dioxide generation source can be efficientlyused. However, since an efficient heat exchanger leads to a costincrease, a simple heat exchange method such as connecting pipes or thelike by a heat transfer member can also produce the effect. It is alsopossible to exchange the heat of the high-temperature carbon dioxide gassupplied from the carbon dioxide generation source 80 between the carbondioxide generation source 80 and the separation tank 13.

The reduction reaction device 81 b has a function of reducing theproduct in the storage part 113. In the reduction reaction device 81 b,a catalyst in which Al₂O₃ or the like carries a metal such as an oxideof copper, palladium, or silver, or Cu—ZnO, Pd—ZnO, or Cu—Zn—Cr is used,for instance, and methanol can be mainly manufactured when hydrogen andCO gas which are raw materials are made to flow at, for example, 150 to300° C. under pressurization. Methanol can also be produced by a liquidphase method that passes the hydrogen and the CO gas in a slurry of theaforesaid catalyst under pressurization. The reduction reaction device81 b includes a heat exchanger for removing heat generated by thereaction, for instance. Further, the reduction reaction device 81 b maybe a device that produces ethanol or nickel by using rhodium or thelike, or produces methane by using ruthenium.

Examples of the product by the reduction reaction in the reductionreaction device 81 b include hydrocarbons such as methane, methanol,ethanol, acetic acid, dimethyl ether, wax, olefin, naphtha, and lightoil. A heat source is not only the carbon dioxide from the carbondioxide generation source but also may include at least part of the heatof the reaction between the reduction product of carbon dioxide andhydrogen, for instance. For example, the mutual heat utilization ofusing part of the reaction heat obtained when methanol is produced bythe reaction of carbon monoxide and hydrogen in the reduction reactiondevice 81 b improves efficiency.

Heat exchange may take place between the carbon dioxide generationsource 80 and the electrolytic solution tank 12. The heat exchange ispossible by providing a heat transfer member 93 connecting, for example,the carbon dioxide generation source 80 and the electrolytic solutiontank 12. The heat transfer member 93 may be provided so as to connectthe flow path 56 and the distiller 81 a, for instance. Alternatively, aheat exchanger or the like may be separately connected.

Heat exchange may take place between the reduction reaction device 81 band the distiller 81 a. The heat exchange is possible by providing aheat transfer member 94 connecting, for example, the reduction reactiondevice 81 b and the distiller 81 a. Further, a heat exchanger or thelike may be separately connected.

In the electrochemical reaction device illustrated in FIG. 7, the heatexchange between the flow path 56 and the distiller 81 a and the heatexchange between the distiller 81 a and the reduction reaction device 81b make it possible to efficiently use and remove the heat of the heatsource.

Incidentally, the electrochemical reaction device illustrated in FIG. 7may include the separation tank 14, the flow path 54, and the flow path55 illustrated in FIG. 4 and so on. Further, an agitator may be providedin an oxygen gas separator to enhance efficiency of separating dissolvedgas by temperature distribution or the like. In this case, the use ofthe carbon dioxide generation source 80, the high-temperature carbondioxide gas obtained from the carbon dioxide generation source 80, theheat generated in the reduction reaction device 81b, or the like as theheat source can improve efficiency. The combination of these heats maybe any, and an operation method for the heat exchange with any of themcan improve efficiency. Further, connecting the flow paths or the likeby the heat transfer member in order to mutually use these heats canimprove efficiency. The storage part 114 a may be connected to at leastone of the storage part 112 and the storage part 115 a via a heattransfer member, for instance.

EXAMPLE Example 1

An electrochemical reaction device having a structure was fabricated.The structure includes a three-junction photoelectric conversion bodywith a 500 nm thickness, a 300 nm thick ZnO layer provided on a firstface of the three-junction photoelectric conversion body, a 200 nm thickAg layer provided on the ZnO layer, a 1.5 mm thick SUS substrateprovided on the Ag layer, and a 100 nm thick ITO layer provided on asecond face of the three-junction photoelectric conversion body.

The three-junction photoelectric conversion body has a firstphotoelectric conversion layer that absorbs light in a short wavelengthrange, a second photoelectric conversion layer that absorbs light in anintermediate wavelength range, and a third photoelectric conversionlayer that absorbs light in a long wavelength range. The firstphotoelectric conversion layer has a p-type microcrystalline siliconlayer, an i-type amorphous silicon layer, and an n-type amorphoussilicon layer. The second photoelectric conversion layer has a p-typemicrocrystalline silicon layer, an i-type amorphous silicon germaniumlayer, and an n-type amorphous silicon layer. The third photoelectricconversion layer has a p-type microcrystalline silicon layer, an i-typeamorphous silicon layer, and an n-type amorphous silicon layer.

An open-circuit voltage when the structure was irradiated with lightusing a solar simulator (AM1.5, 1000 W/cm²) was measured. Theopen-circuit voltage was 2.1 V.

A Ni(OH)₂ layer with a 200 nm thickness was formed as an oxidationcatalyst on the ITO layer on the structure of the three-junctionphotoelectric conversion body by an electrodeposition method usingnickel nitrate. A 500 nm thick gold nanoparticle layer carried by carbonwas formed as a reduction catalyst on the SUS substrate.

The above structure was cut into a square shape and its edge portionswere sealed with a thermosetting epoxy resin. The periphery of thestructure was surrounded by an ion exchange membrane (Nafion (registeredtrademark)), whereby a single sheet-shaped structure was formed. A 10 cmsquare unit was fabricated from the combination of the ion exchangemembrane and a plurality of cells, and ten pieces of the units werearranged in each of the vertical and lateral directions to fabricate a100 cm square photoelectrochemical reaction unit. The sheet-shapedstructure may be formed by, for example, embedding photoelectricconversion cells in a plurality of holes of one ion exchange membranehaving the plural holes. The sheet-shaped structure may be formed byarranging a plurality of structures in each of which a photoelectricconversion cell is embedded in a hole of an ion exchange membrane havingone hole. Ion exchange membranes may be embedded in holes ofphotoelectric conversion cells each having a hole.

This sheet-shaped photoelectrochemical reaction unit is sandwiched by apair of 3 cm thick frames each having a hollow portion with 100 cmlength×100 cm width, and a silicone resin layer was formed between thepair of frames. A window formed of non-reflective glass for solar cellwas fabricated to cover the hollow portion of one of the pair of frames.An acrylic resin plate was formed to cover the hollow portion of theother of the pair of frames. Consequently, a sealed body encapsulatingthe photoelectrochemical reaction unit was fabricated. Flow paths wereprovided on the Ni(OH)₂ layer side and the gold nanoparticle layer sideof the photoelectrochemical reaction unit respectively. As anelectrolytic solution, a 0.5 M aqueous potassium hydrogen phosphatesolution containing saturated carbon dioxide gas was used. A gasrecovery flow path for capturing produced gas was provided in part of anelectrolytic solution tank. Through the above, a photoelectrochemicalreaction module was fabricated. An acrylic vessel with an internalvolume of 30 cm×3 cm×3 cm was connected as a mixing tank to the goldnanoparticle layer side of the module.

This module was immersed in an electrolytic solution tank which was acylindrical glass vessel with a 30 cc volume, and 50 cc/min CO₂ gas wasblown to the electrolytic solution tank to be dissolved in theelectrolytic solution. This electrolytic solution was supplied to thereduction electrode side of the module at a 0.1 cc/min flow rate to becirculated. Further, a potassium borate buffer solution on the oxidationelectrode side was circulated at a 0.1 cc/min flow rate via a buffertank, which was a cylindrical vessel with a 30 cc volume, withoutblowing CO₂.

In the module of the example 1, when A.M.1.5 pseudo sunlight wasradiated from the oxidation electrode side to cause a 0.5 hour reaction,a current value was approximately 1 mA/cm² at an initial stage, butdecreased to 0.4 mA/cm².

In the module of the example 1, when the electrolytic solution tank wasput in ice water to be cooled after the 0.5 hour reaction was caused bythe radiation of the A.M.1.5 pseudo sunlight from the oxidationelectrode side, the current value recovered to approximately 0.7 mA/cm².From this, it is seen that cooling the electrolytic solution containingcarbon dioxide can improve reaction efficiency.

In the module of the example 1, when the A.M.1.5 pseudo sunlight wasradiated from the oxidation electrode side and the flow rate was set to0.2 cc/min, it was possible to make the current decrease time about 1.7times. From this, it is seen that increasing the circulation flow ratecan impede the decrease of the current.

Example 2

A composite substrate (4 cm square) having a 1.5 mm thick SUS substrateconnected to a generator via a lead and a gold-carrying carbon filmprovided on the SUS substrate and carrying 0.25 mg/cm² gold, and aplatinum foil (4 cm square) were prepared. The generator is a simulationdevice of a solar cell. A flow path and a gas flow path were formed oneach of an oxidation electrode side and a reduction electrode side of a5 cm square acrylic frame with a 1 cm thickness. The composite substrateand the platinum foil were enclosed in the frame, an ion exchangemembrane (Nafion 117, 6 cm square) was provided between the compositesubstrate and the platinum foil, and a silicon rubber sheet and anacrylic plate (7 cm length×7 cm width×3 mm thickness) were provided oneach of an outer side of the composite substrate and an outer side ofthe platinum foil, whereby a module sandwiched by these was fabricated.A potassium phosphate buffer solution with pH7 was supplied into themodule. The composite substrate was used as a reduction electrode, theplatinum foil was used as an oxidation electrode, and a silver-silverchloride electrode was used as a reference electrode. Carbon dioxide wasdecomposed by passing a current under a 37 mA: 2.3 mA/cm² conditionusing a galvanostat. This module was immersed in an electrolyticsolution tank which was a cylindrical glass vessel with a 30 cc volume,and CO₂ gas at 50 cc/min was blown into the electrolytic solution tankto be dissolved in the electrolytic solution. This electrolytic solutionwas supplied to the reduction electrode side of the module at a 0.1cc/min flow rate to be circulated. A potassium borate buffer solution onthe oxidation electrode side was circulated at a 0.1 cc/min flow ratevia a buffer tank, which was a 30 cc cylindrical vessel, without blowingCO₂.

In the module of the example 2, when a 0.5 hour reaction was causedunder a 37 mA current and a 0.1 cc/min circulation flow rate, apotential was approximately −1 V at an initial stage, but decreased to−1.4 V.

In the module of the example 2, when the electrolytic solution tank wasput in ice water to be cooled after the 0.5 hour reaction was causedunder the 37 mA current and the 0.1 cc/min circulation flow rate, thepotential recovered to approximately −0.8 V. From this, it is seen thatcooling the electrolytic solution containing carbon dioxide can improvereaction efficiency.

In the module of the example 2, when the flow rate was changed to 0.2cc/min after the 0.5 hour reaction was caused under the 37 mA currentand the 0.1 cc/min circulation flow rate, it was possible to make thepotential decrease time about twice. From this, it is seen thatincreasing the circulation flow rate can impede the decrease of thepotential

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. An electrochemical reaction device comprising: afirst electrolytic solution tank including a first storage part storinga first electrolytic solution containing carbon dioxide and a secondstorage part storing a second electrolytic solution containing water; areduction electrode immersed in the first electrolytic solution; anoxidation electrode immersed in the second electrolytic solution; agenerator connected to the reduction electrode and the oxidationelectrode; a second electrolytic solution tank including a third storagepart storing a third electrolytic solution containing carbon dioxide;and a flow path connecting the first storage part and the third storagepart, wherein a temperature of the third electrolytic solution is lowerthan a temperature of the first electrolytic solution.
 2. The device ofclaim 1, further comprising: a first separation tank including a fourthstorage part storing a fourth electrolytic solution containing carbondioxide and a first gas-liquid separation membrane dividing the fourthstorage part into a plurality of regions; a second separation tankincluding a fifth storage part storing a fifth electrolytic solutioncontaining water and a second gas-liquid separation membrane dividingthe fifth storage part into a plurality of regions; a second flow pathconnecting the first storage part and the fourth storage part; a thirdflow path connecting the third storage part and the fourth storage part;and a fourth flow path connecting the second storage part and the fifthstorage part.
 3. The device of claim 2, further comprising: a carbondioxide generation source containing carbon dioxide having a highertemperature than the temperature of the first electrolytic solution; areduction reaction device reducing a product produced by a reductionreaction of the carbon dioxide; a distiller disposed on the thirdstorage part; a fifth flow path connecting the first storage part andthe carbon dioxide generation source; and a sixth flow path connectingthe third storage part and the reduction reaction device.
 4. The deviceof claim 3, further comprising: a first cooler disposed at the thirdstorage part; a first heater disposed at the second flow path; a secondcooler disposed at the fourth flow path; and a second heater disposed atthe fifth flow path.
 5. The device of claim 3, wherein the devicepertains at least one of heat exchange between the second electrolyticsolution tank and the second separation tank, heat exchange between thefirst separation tank and the second separation tank, heat exchangebetween the reduction reaction device and the second electrolyticsolution tank, heat exchange between the carbon dioxide generationsource and the distiller, or heat exchange between the reductionreaction device and the distiller.
 6. The electrochemical reactiondevice of claim 3, further comprising at least one of a heat transfermember connecting between the second electrolytic solution tank and thesecond separation tank, a heat transfer member connecting between thefirst separation tank and the second separation tank, a heat transfermember connecting between the reduction reaction device and the secondelectrolytic solution tank, a heat transfer member connecting betweenthe carbon dioxide generation source and the distiller, or a heattransfer member connecting between the reduction reaction device and thedistiller.
 7. The device of claim 1, wherein the generator includes aphotoelectric conversion body having a first face connected to thereduction electrode and a second face connected to the oxidationelectrode.
 8. The electrochemical reaction device of claim 1, furthercomprising an ion exchange membrane disposed between the first storagepart and the second storage part.
 9. The device of claim 1, wherein apressure applied to the third electrolytic solution is higher than apressure applied to the first electrolytic solution.
 10. Anelectrochemical reaction device comprising: a first electrolyticsolution tank including a first storage part storing a firstelectrolytic solution containing carbon dioxide and a second storagepart storing a second electrolytic solution containing water; areduction electrode immersed in the first electrolytic solution; anoxidation electrode immersed in the second electrolytic solution; agenerator connected to the reduction electrode and the oxidationelectrode; a second electrolytic solution tank including a third storagepart storing a third electrolytic solution containing carbon dioxide;and a flow path connecting the first storage part and the third storagepart, wherein a pressure applied to the third electrolytic solution ishigher than a pressure applied to the first electrolytic solution. 11.The device of claim 10, further comprising: a first separation tankincluding a fourth storage part storing a fourth electrolytic solutioncontaining carbon dioxide and a first gas-liquid separation membranedividing the fourth storage part into a plurality of regions; a secondseparation tank including a fifth storage part storing a fifthelectrolytic solution containing water and a second gas-liquidseparation membrane dividing the fifth storage part into a plurality ofregions; a second flow path connecting the first storage part and thefourth storage part; a third flow path connecting the third storage partand the fourth storage part; and a fourth flow path connecting thesecond storage part and the fifth storage part.
 12. The device of claim11, further comprising: a carbon dioxide generation source containingcarbon dioxide having a higher temperature than the temperature of thefirst electrolytic solution; a reduction reaction device reducing aproduct produced by a reduction reaction of the carbon dioxide; adistiller disposed on the third storage part; a fifth flow pathconnecting the first storage part and the carbon dioxide generationsource; and a sixth flow path connecting the third storage part and thereduction reaction device.
 13. The device of claim 10, wherein thegenerator includes a photoelectric conversion body having a first faceconnected to the reduction electrode and a second face connected to theoxidation electrode.
 14. The electrochemical reaction device of claim10, further comprising an ion exchange membrane disposed between thefirst storage part and the second storage part.